In dealing with apparatus for the feeding of liquid synthetic resin components, reference may be had to the commonly owned U.S. Pat. No. 4,167,236 and the corresponding German Open Application DE-0S No. 26 13 771, as well as patents which may be found in International Class B29B 5/06.
The aforementioned U.S. patent and its corresponding German application describe an apparatus for the metering, mixing and recirculation of at least two mutually interreactive liquid synthetic resin components, at least one of which may be highly charged with solid particles, generally referred to as a filler, pigment or the like.
Systems of this type may be used, for example, for feeding capral lactam components adapted to produce a nylon upon mixing.
Generally speaking, the apparatus can comprise a supply vessel and a metering piston unit for at least one of these components, a mixing head into which the two components are fed for mutual interaction and mixing and from which the mixture is discharged, and means enabling recirculation of the components from the mixing head so that a static condition in the component path is not generated which may lead to obstruction and interference with the regularity of operations.
The metering piston chamber of the metering piston unit is thus connected by a duct on the one hand with the supply vessel and with a duct on the other hand with the mixing head and the duct between the supply vessel and the metering piston unit can have a controllable valve which operates so that, during the suction stroke or intake stroke of the metering piston unit, the respective component will only pass from the supply vessel or will only be drawn into the metering piston chamber from the supply vessel while, during the pressure stroke, this component will only be displaced to the mixing head.
Generally each of the components may be provided with such a supply vessel, metering piston unit and a control arrangement allowing operation in the manner described.
The apparatus of the aforementioned U.S. patent and its German corresponding application operates such that the metering piston unit during a suction stroke draws the respective component via a line from the respective supply vessel. The latter line can include a check valve so that the suction operation is permitted. During the pressure stroke, however, the check valve has a blocking mode so that the respective component is displaced by the metering piston via a line to the mixing head and, depending upon the position of the latter, either enters the mixing chamber and is discharged as part of a reactive mixture, or is blocked from entry into the mixing chamber and is recirculated. In the earlier systems, moreover, the recirculating component was returned to the respective supply vessel.
Generally the metering piston unit of this system is in continuous operation so that practically continuously a portion of the quantity of the component is fed to the mixing chamber during a stroke of the metering piston. There, this portion of the metered component mixes with the second component and the resulting mixture can be discharged into a mold cavity in which the reaction can be completed.
The metering piston unit can also serve to increase the pressure of the component upstream of the mixing chamber and, when caprolactam is fed to the mixing chamber in the production of nylon, this pressure can amount to about 20 bar. In the production of polyurethane, the pressure of the metered polyisocyanate or di-isocyanate component or the pressure of the polyol component can be 200 bar upstream of the mixing chamber.
Especially when at least one of these components is highly filled, i.e. contains a high proportion of a solid filler, e.g. glass fibers to serve as a reinforcement in the resulting synthetic resin, a significant disadvantage arises in that the line between the supply vessel and the metering piston unit for the highly filled component tends to become clogged. The contents of this line is subjected only during the suction stroke to a comparatively low suction pressure which does not always free the obstruction. The problem is especially pronounced when a comparatively low pressure, say the 20 bar required for caprolactam, is generated at the inlet to the mixing chamber. In this case, the suction pressure can be a maximum of 0.4 bar in practice.